The production of light olefins (ethylene, propylene and butenes) from various hydrocarbon feedstocks utilizes the technique of pyrolysis, or steam cracking. Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules.
In the steam cracking process, it is desirable to maximize the recovery of useful heat from the process effluent stream exiting the cracking furnace. Effective recovery of this heat is one of the key elements of a steam cracker's energy efficiency.
The steam cracking process, however, also produces molecules which tend to combine to form high molecular weight materials known as tar. Tar is a high-boiling point, viscous, reactive material that, under certain conditions, can foul heat exchange equipment, rendering heat exchangers ineffective. The fouling propensity can be characterized by three temperature regimes.
Above the hydrocarbon dew point (the temperature at which the first drop of liquid condenses), the fouling tendency is relatively low. Vapor phase fouling is generally not severe, and there is no liquid present that could cause fouling. Appropriately designed transfer line heat exchangers are therefore capable of recovering heat in this regime with minimal fouling.
Between the hydrocarbon dew point and the temperature at which steam cracked tar is fully condensed, the fouling tendency is high. In this regime, the heaviest components in the stream condense. These components are believed to be sticky and/or viscous, causing them to adhere to surfaces. Furthermore, once this material adheres to a surface, it is subject to thermal degradation that hardens it and makes it more difficult to remove.
At or below the temperature at which steam cracked tar is fully condensed, the fouling tendency is relatively low. In this regime, the condensed material is fluid enough to flow readily at the conditions of the process, and fouling is generally not a serious problem.
One technique used to cool pyrolysis unit effluent and remove the resulting tar employs heat exchangers followed by a water quench tower in which the condensibles are removed. This technique has proven effective when cracking light gases, primarily ethane, propane and butane, because crackers that process light feeds, collectively referred to as gas crackers, produce relatively small quantities of tar. As a result, heat exchangers can efficiently recover most of the valuable heat without fouling and the relatively small amount of tar can be separated from the water quench albeit with some difficulty.
This technique is, however, not satisfactory for use with steam crackers that crack naphthas or feedstocks heavier than naphthas, collectively referred to as liquid crackers, since liquid crackers generate much larger quantities of tar than gas crackers. Heat exchangers can be used to remove some of the heat from liquid cracking, but only down to the temperature at which tar begins to condense. Below this temperature, conventional heat exchangers cannot be used because they would foul rapidly from accumulation and thermal degradation of tar on the heat exchanger surfaces. In addition, when the pyrolysis effluent from these feedstocks is quenched, some of the heavy oils and tars produced have approximately the same density as water and can form stable oil/water emulsions. Moreover, the larger quantity of heavy oils and tars produced by liquid cracking would render water quench operations ineffective, making it difficult to raise steam from the condensed water and to dispose of excess quench water and the heavy oil and tar in an environmentally acceptable manner.
Accordingly, in most commercial liquid crackers, cooling of the effluent from the cracking furnace is normally achieved using a system of heat exchangers, typically transfer line exchangers, a primary fractionator, and a water quench tower or indirect condenser. For a typical heavier than naphtha feedstock, the transfer line heat exchangers cool the process stream to about 593° C. (1100° F.), efficiently generating super-high pressure steam which can be used elsewhere in the process. The primary fractionator is normally used to condense and separate the tar from the lighter liquid fraction, known as pyrolysis gasoline, and to recover the heat between about 93° and about 316° C. (200° F. to 600° F.). The water quench tower or indirect condenser further cools the gas stream exiting the primary fractionator to about 40° C. (100° F.) to condense the bulk of the dilution steam present and to separate pyrolysis gasoline from the gaseous olefinic product, which is then sent to a compressor.
The present invention seeks to provide a simplified method for treating pyrolysis unit effluent, particularly the effluent from the steam cracking of hydrocarbonaceous feeds that are heavier than naphthas. Heavy feed cracking is often more economically advantageous than naphtha cracking, but in the past it suffered from poor energy efficiency and higher investment requirements. The present invention optimizes recovery of the useful heat energy resulting from heavy feed steam cracking without fouling of the cooling equipment. This invention can also obviate the need for a primary fractionator tower and its ancillary equipment.
Heavy feed steam cracking effluent can be treated by using a primary transfer line heat exchanger generating high pressure steam to initially cool the furnace effluent. The surfaces of heat exchanger tubes must operate above the hydrocarbon dew point to avoid rapid fouling, typically an average bulk outlet temperature of about 593° C. (about 1100° F.) for a heavy gas oil feedstock. Additional cooling can be provided by directly injecting a quench liquid such as tar or distillate to immediately cool the stream without fouling. Alternatively, the pyrolysis furnace effluent can be directly quenched, e.g., with distillate, which also avoids fouling. However, the former cooling method suffers from the drawback that only a fraction of the heat is recovered in a primary transfer line exchanger; moreover, in both methods, remaining heat removed by direct quenching is recovered at a lower temperature where it is less valuable. Furthermore, additional investment is required in the downstream primary fractionator where low level heat is ultimately removed, and in offsite boilers which must generate the remaining high pressure steam required by the steam cracking plant.
Accordingly, it would be desirable to recover useful heat from steam cracking furnace effluent in the absence of rapid fouling and absent direct quenching.
U.S. Pat. Nos. 4,279,733 and 4,279,734 propose cracking methods using a quencher, indirect heat exchanger and fractionator to cool effluent, resulting from steam cracking.
U.S. Pat. Nos. 4,150,716 and 4,233,137 propose a heat recovery apparatus comprising a pre-cooling zone where the effluent resulting from steam cracking is brought into contact with a sprayed quenching oil, a heat recovery zone, and a separating zone.
Lohr et al., “Steam-cracker Economy Keyed to Quenching,” Oil & Gas Journal, Vol. 76 (No. 20), pp. 63-68, (1978), proposes a two-stage quenching involving indirect quenching with a transfer line heat exchanger to produce high-pressure steam along with direct quenching with a quench oil to produce medium-pressure steam.
U.S. Pat. Nos. 5,092,981 and 5,324,486 propose a two-stage quench process for effluent from steam cracking, comprising a primary transfer line heat exchanger which functions to rapidly cool furnace effluent and to generate high temperature steam and a secondary transfer line heat exchanger which functions to cool the furnace effluent to as low a temperature as possible consistent with efficient primary fractionator or quench tower performance and to generate medium- to low-pressure steam.
U.S. Pat. No. 5,107,921 proposes transfer line exchangers having multiple tube passes of different tube diameters. U.S. Pat. No. 4,457,364 proposes a close-coupled transfer line heat exchanger unit.
U.S. Pat. No. 3,923,921 proposes a naphtha steam cracking process comprising passing effluent through a transfer line exchanger to cool the effluent and thereafter through a quench tower.
WO 93/12200 proposes a method for quenching the gaseous effluent from a hydrocarbon pyrolysis unit by passing the effluent through transfer line heat exchangers and then quenching the effluent with liquid water so that the effluent is cooled to a temperature in the range of 105° C. to 130° C. (221° F. to 266° F.), such that heavy oils and tars condense, as the effluent enters a primary separation vessel. The condensed oils and tars are separated from the gaseous effluent in the primary separation vessel and the remaining gaseous effluent is passed to a quench tower where the temperature of the effluent is reduced to a level at which the effluent is chemically stable.
EP 205 205 proposes a method for cooling a fluid such as a cracked reaction product by using transfer line exchangers having two or more separate heat exchanging sections.
JP 2001-40366 proposes cooling mixed gas in a high temperature range with a horizontal heat exchanger and then with a vertical heat exchanger having its heat exchange planes installed in the vertical direction. A heavy component condensed in the vertical exchanger is thereafter separated by distillation at downstream refining steps.
WO 00/56841; GB 1,390,382; GB 1,309,309; and U.S. Pat. Nos. 4,444,697; 4,446,003; 4,121,908; 4,150,716; 4,233,137; 3,923,921; 3,907,661; and 3,959,420; propose various apparatus for quenching a hot cracked gaseous stream wherein the hot gaseous stream is passed through a quench pipe or quench tube wherein a liquid coolant (quench oil) is injected.